The present invention is concerned with a planetary wheel advance for a meltable welding cable whose planetary wheels can be clamped against the cable by means of a pressure arrangement and their rotation axes form an angle with the axis of the cable, whereby the planetary wheels are attached to one end of a two arm wheel holder lever which is mounted on a rotating bearing or support and is movable by means of the pressure arrangement as well as being provided with a motor for the rotary motion of the support and the planetary wheels around the welding cable.
The state of the art for planetary wheel advances is exemplified by German Pat. Nos. DRP 514,759; DT-AS 2,125,441; BE-PS 705,047 as well as U.S. Pat. Nos. 1,733,836; 3,014,519 and 3,424,012.
An arrangement is disclosed in DRP 514,759, according to which the rotation axes of the planetary wheels can be adjusted with respect to the axis of the welding cable in order to change the rate of feed of the cable. The centering of the wheels with relation to the cable and also the clamping of the wheels to the cable is accomplished with screws and rubber discs or springs.
With this known arrangement, the disadvantage is that the centrifugal forces occurring with the spinning of the planetary wheels cause lifting of the wheels against the force of the rubber discs or springs. Since the centrifugal forces depend on the rotational speed, that is, on the cable advance rate, it is not assured that wheels are pressed against the electrode cable with a constant pressure during the advance of the electrode cable and that the rate of advance remains constant. But this is precisely an essential prerequisite for assuring trouble free feed of the cable.
With the arrangement according to BE-PS 705,047, in order to adjust the clamping force, a screw is provided by means of which two wheels fastened to levers and pivotable about the center of rotation, are attached. The adjustment of the various clamping forces with the adjusting screw provided inside the advance arrangement is, however, difficult and inaccurate.
According to U.S. Pat. No. 1,733,836, a planetary wheel advance is likewise provided whereby a pair of planetary wheels is supposed to serve for reverse motion of the welding rod, and where, futhermore the actuation of the wheels occurs depending on the arc-voltage. A disadvantage is the expensive and complicated adjustment mechanism for clamping both pairs of wheels on the electrode cable.
U.S. Pat. No. 3,014,519 also shows the complicated construction of the up to now known planetary wheel advances. It can be added to this, that also with this advance, the influence of the centrifugal forces is only to be avoided by means of the springs. This again, has the disadvantage that the spring forces must be designed and dimensioned according to the centrifugal forces expected. As a result of the spring constant and the various magnitudes of centrifugal forces with different rotation rates, the disadvantage exists that the wheels are pressed against the cable with different size forces.
Also the device disclosed in U.S. Pat. No. 3,424,012 is quite complicated in construction. The clamping of the wheels, with this device results by means of springs and screws, whereby a separate adjusting organ is associated with each wheel whereby the centering of the wheels in relation to one another as well as the precise adjustment of clamping forces is made more difficult. It can hereto be added that the spring loaded wheel is again sensitive to centrifugal forces and a precise and constant clamping pressure is thus not assured.
Beyond the above described devices, a planetary wheel advance is known from DT-OS 2,055,721 according to which the wheel holders are supported by one surface of an adjusting nut which can be turned about the welding cable. With this measure, it is accomplished that the centrifugal forces, which occur upon turning the planetary wheels about the welding cable to be advanced and which inhibit the positive locking between electrode cable and wheels, can no longer exercise any influence over the cable advance. This is especially made possible because the wheels are firmly supported during turning by the adjusting nut by means of the slidable wheel holders and as a result, it is avoided that an individual planetary wheel could lift from the cable on account of centrifugal forces. Practice has shown, however, that with such advance mechanisms, the setting of the clamping force in other words the turning of the adjusting nut must constantly be done by hand and thus depending on the skill of the welder the rollers are squeezed against the cable with sometimes more and sometimes less than correct clamping pressure.